The present invention relates to an ink jet printer and an arrangement for promoting the separation of ink and air in an ink jet printer. Aspects of the invention also provide an ink tank for use in an ink jet printer, the ink tank comprising the arrangement for promoting separation of ink and air and optionally also comprising a filter, and a removable module comprising the ink tank.
Continuous ink jet printers are commonly used for printing identification and other variable data on industrial products and packaging. During operation of a continuous ink jet printer, a continuous stream of ink drops is generated and means are provided for deflecting the drops in flight, so that different drops can travel to different destinations. Since the drops are generated continuously, only some of the drops will be required for printing. Accordingly, the drops required for printing are arranged to travel in a direction so that they reach the surface to be printed onto, whereas drops which are not required for printing are arranged to travel to a means, usually known as a gutter, where they are collected. In almost all modern continuous ink jet printers, ink collected at the gutter is returned to an ink tank, from which ink is supplied to the means (sometimes called the ink gun) which creates the stream of ink drops. Such printers are commonly known as continuous ink jet printers because the ink jet is produced even at moments when ink is not required for printing, as opposed to drop-on-demand printers in which the printing process involves producing only the ink drops required to be printed.
Typically, the ink is electrically conductive when wet, and an arrangement of electrodes is provided to trap electric charges on the ink drops and create electrostatic fields in order to deflect the charged drops. The ink gun, the various electrodes and the gutter are fixed in the appropriate spatial relationship in a printhead. Various tanks, pumps, control circuits and the like are housed within a printer body, and the head is usually connected to the body by a flexible conduit carrying fluid lines and electrical wiring, which may be a few meters long.
The ink contains one or more colouring substances together with various other components such as a binder resin, carried in a solvent such as methylethylketone, acetone or ethanol. The solvent is highly volatile, to ensure that the printed ink drops dry quickly. Consequently, the solvent has a tendency to evaporate from the ink during operation of the printer, so that the ink in the ink tank becomes too concentrated. Accordingly, a typical ink jet printer will also have a tank of spare solvent, also housed in the main body, and an arrangement for monitoring ink viscosity directly or indirectly. When the viscosity exceeds a predetermined level, a small dose of solvent will be transferred from the solvent tank into the ink tank to dilute the ink.
In order that the ink collected by the gutter should be conveyed along the gutter line away from the gutter, suction is usually applied to the gutter line from a suction source, typically in the main printer body. The fluid travelling along the gutter line will be a mixture of ink and air. Air inevitably enters the gutter both as a result of the suction applied to the gutter line and because the ink drops moving through the air from the ink gun to the gutter inevitably entrain some air in their path. Therefore the ink returning from the gutter to the ink tank becomes mixed with air.
If this air remains in the ink when the ink is returned to the ink gun, it will tend to disrupt the formation of the ink jet. For example, the ink may be at a pressure of approximately three times atmospheric pressure immediately before it leaves the ink gun through the jet-forming nozzle. At this pressure, any air mixed in with the ink will be substantially compressed. Immediately the ink leaves the nozzle, it will be exposed to atmospheric pressure. This pressure change will cause any air mixed into the ink to expand abruptly, disrupting the jet. Additionally, air bubbles can partially block the nozzle, which may make the ink jet become unstable or non-uniform, which in turn interferes with the break-up of the jet into drops so that the drops are incorrectly deflected. The incorrect deflection both results in incorrect printing and ink contamination of the printhead and/or the surface being printed onto. Partial blockage of the nozzle may also change the direction of travel of the ink jet, causing it to strike components of the printhead. Therefore it is desirable to ensure that the air that gets mixed into the ink as it returns to the ink tank is substantially separated out of the ink before the ink is returned to the ink gun.
The ink also tends to accumulate undesirable particulate matter, such as dried ink particles, dust, and the like. It is desirable to remove this particulate matter from the ink, since it may cause problems, for example by totally or partially blocking the nozzle of the ink gun. The ink may be passed through a filter to remove such material. However, under some circumstances air that is mixed into the ink may pass through the filter, especially if the air is in the form of very small bubbles, and so the filter cannot be relied on to prevent air from remaining in the ink that is returned to the ink gun.